^1,2,3Nicholas E. Timms et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.04.031]
¹The Institute for Geoscience Research (TIGeR), Curtin University, Perth, GPO Box U1987, WA 6845, Australia
²Space Science and Technology Centre, Curtin University, Perth, GPO Box U1987, WA 6845, Australia
³School of Earth and Planetary Sciences, Curtin University, Perth, GPO Box U1987, WA 6845, Australia
Copyright: Elsevier

Hydrothermal activity is a common phenomenon in the wake of impact events, yet identifying and dating impact hydrothermal systems can be challenging. This study provides the first detailed assessment of the effects of shock microstructures and impact-related alteration on the U-Pb systematics and trace elements of titanite (CaTiSiO5), focusing on shocked granite target rocks from the peak ring of the Chicxulub impact structure, Mexico. A >1 mm long, shock-twinned titanite grain preserves a dense network of irregular microcracks, some of which exploit shock twin interfaces. Secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks. In situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS) analysis reveals a mixture of three end-member Pb components. The Pb components are: 1) common Pb, consistent with the Pb isotopic signature of adjacent alkali feldspar; 2) radiogenic Pb accumulated since magmatic crystallization; and 3) a secondary, younger Pb signature due to impact-related complete radiogenic Pb loss. The youngest derived ages define a regression from common Pb that intersects Concordia at 67 ± 4 Ma, in agreement with the established age of 66.04 ± 0.05 Ma for the Chicxulub impact event. Contour maps of LA-MC-ICPMS data reveal that the young ages are spatially restricted to microstructurally-complex domains that correlate with significant depletion in trace elements (REE-Y-Zr-Nb-Mo-Sn-Th) and reduction in magnitude of the Eu/Eu* anomaly. Mapping by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show that patterns of localised element depletion in titanite are spatially related to microcracks, which are enriched in Al. The spatial correlation of ages and trace element abundance is consistent with localised removal of Pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid-mediated element transport process associated with the impact event. Here we interpret the 67 ± 4 Ma U-Pb age to represent hydrothermal Pb-loss in the Chicxulub peak ring in the wake of the impact event. These results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post-impact hydrothermal activity in impact structures elsewhere.

^1,2,3Mingming Zhang,^1,4Enrica Bonato,¹Ashley J. King,¹Sara S. Russell,⁵Guoqiang Tang,^2,3Yangting Lin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13473]
¹Department of Earth Sciences, The Natural History Museum, Cromwell Road, SW7 5BD London, UK
²Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
³University of Chinese Academy of Sciences, Beijing, 100049 China
⁴School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ Glasgow, UK
⁵State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (∆¹⁷O: −25 to −2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (∆¹⁷O: −25 to −20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.

¹Sutton, S.R.,¹Lanzirotti, A.,¹Newville, M.,^2,3Dyar, M.D.,⁴Delaney, J.
Chemical Geology 531, 119305 Link to Article [DOI: 10.1016/j.chemgeo.2019.119305]
¹U. Chicago, IL, United States
²Planetary Science Institute, AZ, United States
³Mount Holyoke College, MA, United States
⁴Rutgers U., NJ, United States

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¹Povinec, P.P.,¹Sýkora, I.,²Ferrière, L.,^2,3Koeberl, C.
Journal of Radioanalytical and Nuclear Chemistry 324, 349-357 Link to Article [DOI: 10.1007/s10967-020-07034-7]
¹Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, 84248, Slovakia
²Natural History Museum, Burgring 7, Vienna, 1010, Austria
³Department of Lithospheric Research, University of Vienna, Vienna, 1090, Austria

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¹Pierre Haenecour,²Christine Floss,³Adrian J. Brearley,^1,4Thomas J. Zega
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13477]
¹Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona, 85721‐0092 USA
²Laboratory for Space Sciences and McDonnell Center for Space Sciences, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
³Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico, 87131 USA
⁴Department of Materials Science and Engineering, The University of Arizona, Tucson, Arizona, 85721‐0012 USA
Published by arrangement with John Wiley & Sons

Our detailed mineralogical, elemental, and isotopic study of the Miller Range (MIL) 07687 meteorite showed that, although this meteorite has affinities to CO chondrites, it also exhibits sufficient differences to warrant classification as an ungrouped carbonaceous chondrite. The most notable feature of MIL 07687 is the presence of two distinct matrix lithologies that result from highly localized aqueous alteration. One of these lithologies is Fe‐rich and exhibits evidence for interaction with water, including the presence of fibrous (dendritic) ferrihydrite. The other lithology, which is Fe‐poor, appears to represent relatively unaltered protolith material. MIL 07687 has presolar grain abundances consistent with those observed in other modestly altered carbonaceous chondrites: the overall abundance of O‐rich presolar grains is 137 ± 3 ppm and the overall abundance of SiC grains is 71 ± 11 ppm. However, there is a large difference in the observed O‐rich and SiC grain number densities between altered and unaltered areas, reflecting partial destruction of presolar grains (both O‐ and C‐rich grains) due to the aqueous alteration experienced by MIL 07687 under highly oxidizing conditions. Detailed coordinated NanoSIMS‐TEM analysis of a large hotspot composed of an isotopically normal core surrounded by a rim composed of ¹⁷O‐rich grains is consistent with either original condensation of the core and surrounding grains in the same parent AGB star, or with grain accretion in the ISM or solar nebula.

¹Bhadeshia, H.K.D.H.
International Materials Reviews 65, 1-27 Link to Article [https://doi.org/10.1080/09506608.2018.1560984]
¹Materials Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom

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¹Wilson, J.W.,¹Marsh, L.A.,¹Van Verre, W.,³Rose, M.C.,²Evatt, G.,²Smedley, A.R.D.,¹Peyton, A.J.
Antarctic Science 32, 58-69 Link to Article [DOI: https://doi.org/10.1017/S0954102019000531]
¹School of Electronic and Electrical Engineering, University of Manchester, Manchester, M13 9PL, United Kingdom
²School of Mathematics, University of Manchester, Manchester, M13 9PL, United Kingdom
³British Antarctic Survey, High Cross, Cambridge, CB3 0ET, United Kingdom

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¹Kadoya, S.,²Krissansen-Totton, J.,¹Catling, D.C.
Geochemistry, Geophysics, Geosystems 21, e2019GC008734 Link to Article [https://doi.org/10.1029/2019GC008734]
¹Department of Earth and Space Sciences, Cross-Campus Astrobiology Program, University of Washington, Seattle, WA, United States
²Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, United States

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¹Jens Barosch,^1,2Dominik C. Hezel,¹Lena Sawatzki,¹Lucia Halbauer,³Yves Marrocchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13476]
¹Department of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49b, 50674 Köln, Germany
²Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD UK
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐lès‐Nancy, 54501 France
Published by arrangement with John Wiley & Sons

Mineralogically zoned chondrules are a common chondrule type in chondrites. They consist of olivine cores, surrounded by low‐Ca pyroxene rims. By serial sectioning porphyritic chondrules from carbonaceous, ordinary, and enstatite chondrites, we demonstrate that the 2‐D textural appearances of these chondrules largely depend on where they are cut. The same chondrule may appear as a porphyritic pyroxene (PP) chondrule when sectioned through the low‐Ca pyroxene rim, and as a porphyritic olivine‐pyroxene (POP) or porphyritic olivine (PO) chondrule when sectioned close or through its equator. Chondrules previously classified into PP/POP/PO chondrules might therefore not represent different types, but various sections through mineralogically zoned chondrules. Classifying chondrule textures into PP, POP, and PO has therefore no unequivocal genetic meaning, it is merely descriptive. Sectioning effects further introduce a systematic bias when determining mineralogically zoned chondrule fractions from 2‐D sections. We determined correction factors to estimate 3‐D mineralogically zoned chondrule fractions when these have been determined in 2‐D sections: 1.24 for carbonaceous chondrites, 1.29 for ordinary chondrites, and 1.62 for enstatite chondrites. Using these factors then shows that mineralogically zoned chondrules are the dominant chondrule type in chondrites with estimated 3‐D fractions of 92% in CC, 52% in OC, and 46% in EC.

¹Tomoya Obase,¹Daisuke Nakashima,¹Tomoki Nakamura,^2,3Keisuke Nagao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13481]
¹Division of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi, 980‐8578 Japan
²Geochemical Research Center, Graduate School of Science, University of Tokyo, Hongo, Bunkyo, Tokyo, 113‐0033 Japan
³Division of Polar Earth‐System Sciences, Korea Polar Research Institute, 26 Songdomirae‐ro, Yeonsu‐gu, Incheon, 21990 Korea
Published by arrangement with John Wiley & Sons

We measured concentrations and isotopic ratios of noble gases in the Rumuruti (R) chondrite Mount Prestrud (PRE) 95410, a regolith breccia exhibiting dark/light structures. The meteorite contains solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated a heliocentric distance of its parent body, a cosmic‐ray exposure age on the parent body regolith (parent body exposure age), and a cosmic‐ray exposure age in interplanetary space (space exposure age) of the meteorite. Assuming a constant solar wind flux, the estimated heliocentric distance was smaller than 1.4 ± 0.3 au, suggesting inward migration from the asteroid belt regions where the parent body formed. The largest known Mars Trojan 5261 Eureka is a potential parent body of PRE 95410. Alternatively, it is possible that the solar wind flux at the time of the parent body exposure was higher by a factor of 2–3 compared to the lunar regolith exposure. In this case, the estimated heliocentric distance is within the asteroid belt region. The parent body exposure age is longer than 19.1 Ma. This result indicates frequent impact events on the parent body like that recorded for other solar‐gas‐rich meteorites. Assuming single‐stage exposure after an ejection event from the parent body, the space exposure age is 11.0 ± 1.1 Ma, which is close to the peak of ~10 Ma in the exposure age distribution for the solar‐gas‐free R chondrites.